Integrated interfacial design of covalent organic framework photocatalysts to promote hydrogen evolution from water

Attempts to develop photocatalysts for hydrogen production from water usually result in low efficiency. Here we report the finding of photocatalysts by integrated interfacial design of stable covalent organic frameworks. We predesigned and constructed different molecular interfaces by fabricating ordered or amorphous π skeletons, installing ligating or non-ligating walls and engineering hydrophobic or hydrophilic pores. This systematic interfacial control over electron transfer, active site immobilisation and water transport enables to identify their distinct roles in the photocatalytic process. The frameworks, combined ordered π skeletons, ligating walls and hydrophilic channels, work under 300–1000 nm with non-noble metal co-catalyst and achieve a hydrogen evolution rate over 11 mmol g–1 h–1, a quantum yield of 3.6% at 600 nm and a three-order-of-magnitude-increased turnover frequency of 18.8 h–1 compared to those obtained with hydrophobic networks. This integrated interfacial design approach is a step towards designing solar-to-chemical energy conversion systems.

to develop efficient and robust photocatalysts to enable H 2 production from water. We found that photocatalysts that merge ordered π skeletons, ligating walls and hydrophilic channels can achieve a H 2 evolution rate over 11 mmol g -1 h -1 , a quantum yield of 3.6% at 600 nm and a turnover frequency of 18.8 h -1 which is increased by more than three orders of magnitude from the results obtained with hydrophobic networks. Remarkably, the photocatalyst is stable to work with nonnoble metal co-catalyst under a wide range of light from 300 nm to 1000 nm. These insights open the way to actionable photocatalysts for green fuel production.

Design principle
Photocatalytic hydrogen generation from water involves a series of continuous photochemical events from light harvesting to charge separation and electron transfer, as well as the water delivery to the catalytic centres. How to merge these physical and chemical processes into one material in a seamless manner is key to hydrogen evolution. To address this fundamental key issue, we predesigned and constructed π skeletons to be ordered or amorphous, walls to be ligating or non-ligating and pores to be hydrophilic or hydrophilic, to constitute different combinations of molecular interfaces into the photocatalysts (Fig. 1a). The combination of these different molecular interfaces leads to the generation of a series of photocatalysts that possess distinct components and structures (Fig. 1b). This systematic interfacial design enables to identify their roles in the photocatalytic process and leads to the finding of efficient photocatalysts and new insights on photo-to-chemical energy conversion.
The hydrophilic ZnP-Pz-PEO-COF with pyrazine ligating sites was synthesised in 96% yield by the reaction of ZnP-Pz-DHTP-COF with 1bromo-2-(2-methoxyethoxy)ethane in the presence of K 2 CO 3 in DMF at 85°C (Fig. 1c-e). Similarly, ZnP-Pz-PEO-POP and ZnP-TP-PEO-COF were prepared in 98% and 96% yields by reacting ZnP-Pz-DHTP-POP and The integrated interfacial design strategy for constructing π skeleton, wall and pore to merge three different interfaces, i.e., electron flow, active site ligating and water transport, into the photocatalysts to promote photocatalytic H 2 production from water.

Band gap structure
The electronic diffuse reflection absorption spectra were investigated to reveal light absorption and optical band gap. All samples exhibited a broad and strong absorption band from 300 nm to over 1000 nm in the visible and near-infrared regions with a maximum absorbance at 612 nm, demonstrating a high light-harvesting efficiency (Fig. 5a). The ZnP-Pz-DHTP-COF (Fig. 5a, red curve) displayed a slightly blue-shifted absorption edge at 706 nm compared to ZnP-Pz-COF at 733 nm (Fig. 5a, black curve). The ZnP-Pz-PEO-COF exhibited a red-shifted absorption edge at 739 nm (Fig. 5a, Table 2).

Photocatalytic activity
Photocatalytic hydrogen evolution experiments were conducted in systems with [Mo 3 S 13 ] 2-@ZnP-Pz-PEO-COF catalyst (10 mg) and sacrificial donor in water (10 mL), upon irradiation with a 300 W Xenon lamp (λ > 420 nm). Firstly, ascorbic acid (50 mM) was used as a Article https://doi.org/10.1038/s41467-023-35999-y sacrificial reagent, H 2 was generated smoothly at a rate of 1.38 mmol g -1 h -1 (Fig. 5c, blue curve). Impressively, the H 2 evolution rate greatly increased to 7.8 mmol g -1 h -1 when ascorbic acid was replaced with lactic acid (10 vol%) (Fig. 5c, red curve). Moreover, different lactic acid contents of 5 vol%, 15 vol% and 20 vol% were investigated to evaluate the H 2 production. As a result, a lactic acid concentration of 15 vol% yielded the highest H 2 evolution rate (Fig. 5d). To exclude the effect of pH values and concentrations between ascorbic acid and lactic acid, we adjusted the pH value of 1.17 M ascorbic acid to 1.46 with diluted HCl aqueous solution, which presented the same concentration and pH value of 15 vol% lactic acid. The resultant H 2 evolution rate was only 1.41 mmol g -1 h -1 (Supplementary Figs. 14 and 15 (Supplementary Figs. 16a  and 17).
Remarkably, [Mo 3 S 13 ] 2-@ZnP-Pz-PEO-COF is comparable to or even higher than those of the state-of-the-art Pt-based systems (Supplementary Table 6). Moreover, [Mo 3 S 13 ] 2-@ZnP-Pz-PEO-COF is much far superior in the rate which is two orders of magnitude as high as those of reported non-noble metal-based systems. For example, the Co-based N 2 -COF and COF-42 only work in CH 3 CN/H 2 O (4/1 in vol) to show a rate of only 0.782 and 0.163 mmol g -1 h -1 , respectively 9,11 , while the Ni-based TpDTz COF 10 and Mo-based EB-COF 31 in water result in a rate of 0.941 mmol g -1 h -1 and less than 2 mmol g -1 h -1 , respectively.

Exciton binding energy
To reveal insights on the separation and transport nature of exciton and charge carriers, we carried out temperature-dependent photoluminescence spectroscopy upon excitation at 580 nm. The luminescence intensity of crystalline ZnP-Pz-PEO-COF decreased gradually as the temperature was raised from 77 to 298 K (Fig. 6a), owing to the progress of thermally activated nonradiative recombination process 32 . The exciton binding energy of ZnP-Pz-PEO-COF was calculated to be 82 meV (Fig. 6b), which is higher than the thermal ionisation energy (26 meV), indicating that the transfer of photogenerated electron-hole pairs is favourable to be excitons rather than to be free electrons and holes 32 . The temperature-dependent fluorescence spectra of amorphous ZnP-Pz-PEO-POP (Fig. 6c) presented similar tendency to ZnP-Pz-PEO-COF. The exciton binding energy of ZnP-Pz-PEO-POP was 92 meV (Fig. 6d). The lower binding energy of ZnP-Pz-PEO-COF demonstrated that the excitons of ZnP-Pz-PEO-COF are easier to dissociate than those of ZnP-Pz-PEO-POP owing to the advantage of ordered π skeletons of crystalline ZnP-Pz-PEO-COF. These results are in accordance with the better photocatalytic activity of [Mo 3 S 13 ] 2-@ZnP-Pz-PEO-COF.

Discussion
Our studies on elucidating the key fundamental interfaces for photoinduced hydrogen evolution from water with non-noble metal catalytic centres unambiguously reveal the necessary structures for photocatalysts and lead to the finding of the best photocatalysts. By setting the structural parameters on skeleton for electron flow, coordination sites for ligating metal centre and nanochannels for mass transport, we predesigned the interfaces for each process and integrated them into the framework materials. Remarkably, the change of structures for these key processes leads to a profound effect on the photocatalytic reaction. Changing the skeleton from amorphous polymer to crystalline framework greatly improves the photocatalytic activity by facilitating electron flow. Changing non-ligating walls to nitrogen-rich ligating pore walls improves the loading of metal centres onto the proximate location on the skeleton, so that photogenerated electrons can be quickly transported from the skeleton to the reaction centre, greatly improving the photocatalytic efficiency. A systematic changing of nanochannels from hydrophobic to hydrophilic enables to construct a wide structural spectrum to facilitate the water delivery to the reaction centre and greatly promotes the photocatalytic reaction. Through comparative studies on counterpart systems with amorphous, hydrophobic and non-ligating walls, the role of each process involved in the photocatalytic reaction becomes clear. The COFs, combined crystalline skeleton, hydrophilic channels and ligating walls, can merge electron transfer, electron flow and mass transport into the photocatalytic cycle to achieve the highest performance. In this sense, this interfacial design approach is a step towards designing photocatalysts via bottom-up structural control.
In summary, we have developed an integrated interfacial design strategy for constructing active and robust photocatalysts for hydrogen production from water. We designed the photocatalyst by engineering three distinct interfaces to facilitate electron flow and water transport to the reaction centre: (1) For the π-electronic interface, the π skeleton was designed to efficiently harvest photons extended to 1000 nm and build ordered π skeletons to promote a seamless electron transfer from the antennae to the catalytic centres; (2) for the nonnoble metal immobilisation interface, the walls were installed with desired ligating sites; and (3) for the mass transport interface, the nanopores were engineered to be hydrophilic to facilitate water delivery to the reaction centres. Remarkably, the resultant photocatalysts work with a non-noble metal co-catalyst and enable the use of a wide range of photons to achieve high evolution rate, quantum yield and turnover frequency, which are far superior to those obtained with the state-of-the-art noble metal platinum systems. These results disclosed unprecedented insights that integrated interfacial design is key to photocatalytic hydrogen evolution. We envision that our approach is widely applicable to other photocatalytic systems and open the way to actionable solar-to-chemical energy conversion and green fuel production.

ZnP-Pz-COF
An o-DCB/dioxane (1/1 in vol, 1 mL) mixture of ZnP (14.6 mg, 0.02 mmol) and pyrazine-2,5-dialdehyde (5.4 mg, 0.04 mmol) in the presence of acetic acid (6 M, 0.1 mL) was degassed in a Pyrex tube (10 mL) by three freeze-pump-thaw cycles. The tube was sealed off and heated at 120°C for 3 days. The precipitate was collected by filtration, washed with THF and subjected to Soxhlet extraction with THF for 1 day. The powder was collected and dried at room temperature under vacuum overnight to give ZnP-Pz-COF in an isolated yield of 88%.

ZnP-Pz-DHTP-POP
An o-DCB (1 mL) of ZnP (14.6 mg, 0.02 mmol), pyrazine-2,5-dialdehyde (2.7 mg, 0.02 mmol), and 2,5-dihydroxyterephthalaldehyde (3.3 mg, 0.02 mmol) in the presence of acetic acid (6 M, 0.1 mL) was degassed in a Pyrex tube (10 mL) by three freeze-pump-thaw cycles. The tube was sealed off and heated at 120°C for 3 days. The precipitate was collected by filtration, washed with THF and subjected to Soxhlet extraction with THF for 1 day. The powder was collected and dried at room temperature under vacuum overnight to give ZnP-Pz-DHT-POP in an isolated yield of 81%.

ZnP-TP-DHTP-COF
An o-DCB/n-butanol (1/1 in vol, 1 mL) mixture of ZnP (14.6 mg, 0.02 mmol), terephthalaldehyde (2.7 mg, 0.02 mmol) and 2,5dihydroxyterephthalaldehyde (3.3 mg, 0.02 mmol) in the presence of acetic acid (6 M, 0.1 mL) was degassed in a Pyrex tube (10 mL) by three freeze-pump-thaw cycles. The tube was sealed off and heated at 120°C for 3 days. The precipitate was collected by filtration, washed with THF and subjected to Soxhlet extraction with THF for 1 day. The powder was collected and dried at room temperature under vacuum overnight to give ZnP-TP-DHTP-COF in an isolated yield of 85%.
[Mo 3 S 13 ] 2-@ZnP-Pz-COF A (NH 4 ) 2 Mo 3 S 13 sample (29 mg, 0.040 mmol) in methanol (10 mL) was sonicated for 30 min and added with ZnP-Pz-COF (50 mg, 0.106 mmol pyrazine unit). The mixture was sonicated for 30 min and stirred at 75°C for 24 h. After cooled to room temperature, the precipitate was collected by centrifugation at 5724×g for 5 min and washed with methanol for three times. The [Mo 3 S 13 ] 2-@ZnP-Pz-COF was collected and dried at room temperature under vacuum overnight. The content of Mo was 6.21%.

Photocatalytic hydrogen evolution
Typically, the photocatalyst (10 mg) was dispersed in an aqueous solution (10 mL) with sacrificial donor (ascorbic acid or lactic acid). The suspension was then bubbled with nitrogen to remove the residual oxygen before sealed in a quartz flask. The photocatalytic hydrogen evolution was carried out by irradiating the mixture with a 300-W Xenon lamp (MAX-303, Asahi Spectra, Japan) and a 420 nm cutoff filter. The amount of hydrogen was measured by using gas chromatograph (Aglient 7890 A, TCD, 13 X columns, S-6 Ar carrier).

Data availability
All data are available in the main text or the supplementary information.